Electrolytic cell for production of aluminum and method of making the same



3,400,061 ALUMINUM Sept. 3, 1968 R. A LEWIS ET AL ELECTROLYTIC CELL FOR PRODUCTION OF AND SAM METHOD OF MAKING Filed Nov. 2l, l

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United States Patent ELECTROLYTIC CELL FOR PRODUCTION OF ALUMINUM AND METHOD OF MAKING THE SAME Robert A. Lewis, Los Altos, and Richard D. Hildebrandt, Cupertino, Califi, assignors to Kaiser Aluminum & Chemical Corporation, Oakland, Calif., a corporation of Delaware Filed Nov. 21, 1963, Ser. No. 325,228 20 Claims. (Cl. 204-67) ABSTRACT OF THE DISCLOSURE This disclosure relates to replacing the substantially flat carbon-lined bottom of a conventional electrolytic cell with at least one drained cathodic structure wettable by molten aluminum. The upper portion or molten constituent contacting surface of the cathodic structure comprises a mixture of refractory-hard metal and at least about percent carbon. The composite cathodic material is characterized by being wettable by molten aluminum and having a high degree of dimensional stability.

This invention relates to a novel electrolytic cell and to a method of operating same. The invention also relates to a new cathodic structure useful in the electrolytic cell and to a method of making the cathodic structure. More particularly, the invention relates to a new electrolytic cell useful in the preparation of aluminum and having a drained and wetted cathode.

The electrolytic cell in common use today for the prep aration of aluminum is of the classic Hall-Herou'lt design and utilizes carbon anodes and a substantially fiat carbon lined bottom which functions as part of the cathodic system. An electrolyte is used in the production of aluminum by electrolytic reduction of alumina which consists primarily of molten cryolite with dissolved alumina and which may contain other materials such as fiuorspar. Molten aluminum resulting from the reduction of alumina accumulates at the bottom of the receptacle forming the electrolytic cell as a molten metal pool over the carbon lined bottom and acts as a liquid metal cathode. The carbon anodes extend into the receptacle from above and make contact with the electrolyte. Current collector bars, usually of steel, are embedded in the carbon lined bottom and complete the connection to the cathodic system.

Electro-magnetic effects and bath circulation cause the liquid metal cathode to vary in thickness and necessarily restricts reduction of the interpolar spacing, i.e., anodecathode distance. Since power is lost to the electrolyte interposed between the anode and cathode, restrictions on narrowing the anode-cathode spacing, also restrict the achievement of maximum power efficiency and limit the ability to improve electrolytic cell operation.

The present invention is an improvement over electrolytic cell designs of the type described. According to the invention the substantially flat carbon lined bottom is replaced with at least one drained cathodic structure wettable by molten aluminum. The upper portion or molten constituent contacting surface of the cathodic structure is comprised of a novel cathode material which possesses good electrical conductivity, good dimensional stability under cathodic conditions in an electrolytic cell and is wettable by molten aluminum. The novel cathodic mixture is referred to herein as composite cathodic mixture and comprises a mixture of refractory hard substance" and at least about 5% carbon. The composite cathodic material is characterized by being wettable by molten aluminum and having a high degree of dimensional stability. The dimensional stability is described by a maximum cathodic expansion and the composite cathode maice terial according to the invention has a cathodic expansion of less than about 3% and preferably less than 1.5%. Materials having unduly high cathodic expansion, e.g. above about 3%, do not possess the degree of stability necessary for electrolytic cells operating at short interpolar distances.

The cathodic expansion as referred to in the specification and claims is a measurement determined by the following testing procedure:

Samples of material to be tested are prepared as cylinders of from 1" to 2" in diameter and from 3" to 6" in length by compaction. The force of compaction employed in making the samples is parallel to the longitudinal axis of the samples. The samples are gradually heated to 985 C. :20 and immersed under an inert atmosphere a known depth into an electrolyte which is also maintained at this temperature for the duration of the test. With the sample as a cathode, direct current is passed through it equivalent to 6 amps per square inch of immersed side surface area of the sample for two hours. After the electrolyzing period the sample is immediately cleaned of adhering bath on the side surfaces. The percentage increase in average diameter is reported as the percent cathodic expansion for the material. The electrolyte used in the test has the following composition: Sodium cryolite, 62.5%; potassium fluoride 5.7%; sodium fluoride 25.2% and alumina, 6.6%. Sufficient electrolyte should be used so that the alumina content does not drop below 3% during the test. The cathodic expansion measured by this testing procedure on individual samples may vary within :t10% of the average. cathodic expansion determined from tests of several samples of the same material.

As used herein, the term refractory hard substance is defined as a material which: (1) is wettable by molten aluminum under electrolytic cell operating conditions, (2) has low solubility in molten aluminum and molten cryolite, (3) has at least moderately good electrical conductivity and, (4) is substantially dimensionally stable in a cathodic structure in an electrolytic cell. Refractory hard substances, according to the invention, include compatible refractory hard metals and mixtures of refractory hard metals as well as mixtures containing refractory hard metals and aluminum compounds such as borides, nitrides, carbides, etc., compounds of rare earth metals, chromium, and combinations of the above. As known in the art the expression refractory hard metals refers to the carbides, borides, silicides and nitrides of the transition metals in the fourth to sixth groups (Hubbards Periodic Chart of the Atoms). The preferred refractory hard substances include the borides of titanium and zirconium.

Commercially available refractory hard substances frequently contain up to /2% carbon as impurities. In the art this is considered pure refractory hard substance. A significant contribution of this invention is that the amount of refractory hard substance in a cathodic structure may be reduced virtually without sacrifice of the refractory hard substances function. Moreover, there are many advantages provided by the carbon component. For example, the presence of carbon in the compisitions of this invention increases the strength of overall cathodic structure and in many respects improves its electrical characteristics.

In an electrolytic cell having a drained and wetted cathode according to the invention, dimensional stability of the cathode structure is essential and critical for its successful operation. Electrolytic cells using drained and Wetted cathodes theroretically represent a great improvement over conventional cells. By draining the cathode surface so that only a thin film of molten aluminum remains in contact with the cathode and functions, as part of the electrical circuit, it is possible to employ very short anode-cathode spacing and at the same time maintain high current efiioiency. Power losses can be reduced by decreasing electrical resistance in the cell. Electrical resistance can be decreased without sacrificing current efiiciency by decreasing the interpolar distance and thereby decreasing voltage loss due to electrolyte resistance (since less electrolyte is interposed between the anode and the cathode). Aluminum which is produce in the drained and wetted cathode cell by the electrolysis of alumina is drained off the cathode surface so that only a thin, substantially uniform film of molten metal remains thereon since the surface is wettable by molten aluminum, i.e. molten aluminum adheres as a liquid to the solid surface. Molten aluminum drained from the cathode surface collects in a pool or well in a molten metal collection area located so that the molten metal pool is not an essential part of the electrical system, i.e. the molten metal pool is not essential for conducting cathode current from the cell and the well may be periodically tapped dry of aluminum, if desired, without adversely affecting cell operation in any way.

In order to obtain maximum benefit of the improve men-ts potentially available from drained and wetted cathode cells it is necessary for such cells to be capable of operating at relatively short anode-cathode distances so that operating voltage and power requirements can be reduced. However, a serious difficulty in operating at short interpolar distances is the instability under cell operating conditions of common cathode structural materials. Another requirement in addition to dimensional stability to enable short interpolar distances to be used is that the cathode surface must be wettable by molten aluminum under cell operating conditions. If the cathode surface is not wet by molten aluminum, metal produced from the electrolysis of alumina remains in the form of small droplets or globules which possess a high surface area. The fine droplets vare highly reactive and, since the alumina reduction reaction is reversible, the highly reactive droplets would back react with electrolyte and/or anode gases. Consequently, the etficiency of the electrolytic operation in a drained cathode cell with a non-wettable cathode surface would be so low as to render the process impractical. Conventional cells utilizing a carbon cathode which is not wet by molten aluminum under cell operating conditions can be operated properly only by maintaining a large molten metal pool in contact with the carbon surface so that the droplets of molten metal produced can collect in the pool or layer which has a considerably smaller surface area than the droplets and which has an adequately low back reaction potential. However, as indicated above, maintaining a relatively thick molten metal layer as in conventional cells precludes the use of short tinterpolar distances because of the variation in thickness in the liquid metal cathode brought about by electromagnetic effects, etc., and the inability to maintain predetermined, small anodecathode spacing without risk of arcing or short-circuiting.

The drained and wetted cathode cell of the invention avoids the aforementioned difficulties by providing a cathode structure which is both adequately dimensionally stable and which is wettable by molten aluminum under cell operating conditions. Since the cathode surface of this electrolytic cell is wet by molten aluminum a high current efiiciency can be maintained at short interpolar distances because the aluminum deposits as a molten metal film on the cathodic structure and drains into a collection zone. Thus, a characteristic of the invention is the ability to achieve high current efficiency while operating at interpolar distances considerably less than heretofore had been practical in alumina reduction cells. For example, where conventional carbon bottom Hall-Heroult type cells typically use anode cathode spacing of 1 /2 to 2 /2 inches, electrolytic cells according to the invention can be operated at interpolar distances of /2 to 1 inch or less and still maintain current efficiencies of or greater. In the art it is commonly believed that high current efficiencies, i.e. above 80%, could only be achieved at increased anode-cathode distances. The ability of the electrolytic cell according to the invention to provide high current efficiency at low anodecathode distances is a distinct indication of the value of the invention.

One method of draining the cathode surface so as to leave only a substantially thin film thereover is to slope the cathodic structure so that molten aluminum which is produced can run off the cathode surface in a predetermined manner. Of course, other techniques for draining the cathode surface may be employed within the purview of the invention. For example, the cathode surface may be drained also by providing a porous cathodic structure wherein molten metal can pass through the pores to a collection zone. Similarly, a horizontal cathode surface equipped with inclined channels, passages, grooves, etc. which permit molten aluminum to flow in a predetermined manner may be used. The presently preferred arrangement includes sloping cathodic surfaces for drainage and an example will be presented with this presently preferred embodiment. However, it is understood that any arrangement providing for the drainage of molten aluminum from the cathode surface may be used which also employs in combination at least one molten constituent contacting surface which comprises the composite cathode material according to the invention.

Conventional cells may be readily modified to embody the invention by providing pedestals on the cell bottom under the anodes which have sloping upper surfaces comprising the composite cathode mixture and in which anodes are positioned with respect to the sloping cathode surface. A composite cathode mixture according to the invention may be employed as merely the upper surface of the pedestal or the entire pedestal may be formed of the material. The mixture may be prebaked prior to employment in the cell or may be baked in situ in position. Shapes composed of the composite mixture may be produced in a separate facility and effectively assembled within the cell to provide a drained cathode surface as described above. The composite cathode mixture itself may be produced in a one-step high temperature calcining process by adding the ingredients e.g. appropriate oxides such as TiO ZrO bon'de forming mixtures, etc., to form the refractory hard substance together with carbon or the mixture may be made by preparing the refractory hard substance and the carbonaceous material as a physical admixture. A composite cathode material may be rammed into place in the cell bottom in a way similar to that now employed in conventional carbon bottom lining operations or fabricated forms can be used. Similarly monolithic or block formed cathodic structures can be used to construct an electrolytic cell according to the invention.

An electrolytic cell illustrating one embodiment of the invention is shown in the single figure of the drawing which is a front elevation view partly in section.

Electrolytic cell 10 comprises an outer shell 18 generally defining a receptacle. Shell 18 is insulated to preserve heat. The shell may be insulated in any suitable manner, as for example with an alumina lining 22 and an additional refractory brick lining 24. A carbon bottom 26 may be used which if desired may be similar to carbon linings in use in conventional cells. The bottom of the cell, however, is constructed so as to provide a drained cathode surface. In the embodiment shown in the drawing, draining is accomplished by sloping the cathode surface so that molten metal will flow across the surface to a molten metal collecting zone or well provided for that purpose. In the drawing the cathode surfaces are shown sloping toward a central molten metal collecting well 32. However, it is apparent that the collection zone can be located in any suitable position which does not interfere with the inclined, electrode structures (anode and cathode surfaces) and so the body of collected molten aluminum does not form an essential part of the electrical circuit.

The composite cathode material is disposed in a layer so that it constitutes the upper portion or molten constituent contacting surface of the cell bottom. Since in the drained cathode cell design no conventional liquid metal cathode pad is used the construction of the cell bottom should be arranged so that the molten constituent contacting surface of composite cathode material extends into the molten electrolyte although, of course, during cell operation a substantially thin layer or film of molten aluminum generally covers the drained and wetted cathodic structure.

In the embodiment of the invention wherein a sloping cathode design is used, the composite material is sloped or inclined from the horizontal so as to provide at least one sloping surface (two in the embodiment shown in the drawing) extending downwardly. The slope of the cathode surface need be only slight enough to enable molten aluminum to run olf into the collecting zone. A slope of 2 has been found to be satisfactory in some arrangements, although more severe inclinations may be used. During cell operation carbon anodes 36 have similarly sloping surfaces 34 which are substantially parallel with the sloping surface of the composite cathode material 30. Car bon anodes 36 nee-d not be fabricated with the sloping surface but, conventional horizontally surfaced anodes may be used, the lower surface of which during cell operation, burns off to a slope conforming to the inclination of the cathode surface.

A conventional electrolyte containing cryolite, alumina, and additives if desired may be employed in the electrolytic cell. Various cryolite forms may be used. During the cell operation, electrolyte 33 fills the space between the sloping cathode and anode surfaces. Alumina dissolved in the electrolyte is reduced and molten aluminum 28 forms a film 27 on the composite cathode material as it runs off and collects in the well or collecting zone provided. A crust forms over the electrolyte. Current collector bars 42 which may be embedded in the carbon bottom or connected to the composite cathode material in any suitable manner serve to complete the circuit by connection to a cathodic bus system (not shown). Various means for withdrawing current lfl'OIl'l the cathode material may be employed in lieu of the collector bar arrange ment disclosed in the drawing. For example, leads may be provided directly to the cathode material either through the cell wall or from the cathode material through the carbon bottom to the collector bars. Such leads may be composed of graphite, refractory hard substance or other suitable electrically conductive material. However, in any arrangement employed the method of operation according to the invention involves passing current from an anode through the electrolyte containing dissolved aluminum compound causing a substantially thin fllm of aluminum to form on the cathode structure, through the substantially thin film of molten aluminum, and then through the composite cathode material to the cathodic current collection system. The body of collected molten aluminum does not form an essential part of the electrical circuit as in conventional cells and no liquid metal cathode is employed except for the substantially thin film of molten aluminum on the drained and wetted cathode surface.

As discussed above the success of a drained and wetted cathode cell depends greatly upon the material used to construct the drained cathode structure. Carbon has been use-d in conventional cells at the cathodic bottom of the cell because of its electrical properties and its'resistance to deterioration under cell operating conditions. The composite cathode material used in the electrolytic cell of the invention possesses electrical properties generally superior to carbon. In a-didtion, however, the composite cathode material is considerably more stable under cell operating conditions and is wet by molten aluminum. Stability of the cathode structure is measured by the amount of dimensional change, i.e. expansion, that the structure experiences under cell operating conditions. It has been postulated that expansion of the cathodic carbon bottom in electrolytic cells occurs in part by penetration of sodium released at the cathode surface into the crystal lattice structure of the carbon bottom.

Drained cathode cell arrangements using short anodecathode distances cannot tolerate severe dimensional changes because of the likelihood of such changes interrupting the electrolytic operation by short circuiting or by generating excessive heat, etc., and interfering with the production of aluminum. Thus, the composite cathode material, according to the invention, accounts to a large measure for the success of the drained and wetted cathode electrolytic cell of the invention.

The superior properties of the composite cathode material are demonstrated below in Table l which indicates electrical resistivity, cathodic instability and wettability by molten aluminum. The composite materials whose properties are compared in Table l have been produced by the preferred procedure for preparing composite cathode material. A composite mixture having the required properties can be prepared by mixing carbon calcined at temperatures of at least about 1500 C. and refractory hard substance and lforming same into suitable bodies. In the preferred embodiment, high temperature calcined carbon is mixed with a compatible refractory hard substance and a carbonaceous binder and the mixture then baked at a temperature of at least about 900 C. A binder is preferably used with the green mixture. In general, 10% to 20% by weight of pitch as binder is preferred. Composite cathode material having less than about 3% cathodic expansion can be prepared using carbon which has been subjected to a high temperature treatment above about 1500" C. to improve its dimensional stability. The preferred heat treating temperature is about 1600"- 2000 C.

The baking step is used as necessary to provide composite cathode material in a rigid, coherent form. Where baking is employed it may be accomplished in the cell or the mixture can be baked in shapes which are then used to construct the interior of the electrolytic cell. The electrical properties of the composite cathode material will improve with higher baking temperatures. In Table 1 the properties after baking of several composite cathode compositions are listed together with the properties of an average carbon cell lining composition used in conventional cells. The data illustrates an additional discovery that composite cathode mixtures containing calcined petroleum coke are superior to composite mixtures containing calcined anthracite coke. In the interests of accuracy and comparison, the refractory hard substance component of the composite cathode mixtures in Table 1, except for sample 2, is the same (a blend of 70% TiB; and 30% TiC). In sample 2 titanium carbide alone was used as the refractory hard substance component of the composition. It is understood however that the examples in the table are illustrative only and other compatible refractory hard substances possessing the necessary properties may be employed in varying proportions with a carbonaceous material within the purview of the invention so long as the resulting mixture is wettable by molten aluminum and has a cathodic expansion of less than 3%. The dimensional instability of an average cell lining material formed of carbon which has been calcined at about 1200 C. is 6 /2% (expansion). The composite mixture has considerably less tendency for dimensional change. Furthermore, although carbon cell linings are not wet by molten aluminum each of the composite cathode material compositions is at least partially and generally substantially or entirely wet by molten aluminum under cell operating conditions. It will also be observed that the improvement in dimensional stability and wettability is obtained without loss of electrical properties.

7 8 Thus, electrical resistivity of the composite cathode mawetted cathode surface includes disposing a mixture of terial is generally at least as good or superior to the 1020% carbonaceous binder and composite cathode maresistivity of an average cell lining composition. In many terial consisting essentially of high temperature calcined cases the resistivity of the refractory mixture is less than carbon and refractory hard substance over a carbon botcarbon. tom as shown in the drawing with a slope of desired mag- In the specification and claims the amounts of matenitude and thereafter heating the mixture in place to a rials are in weight percentages. temperature of at least about 900 C. during baking of Wherever used in this application, the term carbon, the carbon bottom. A base of high temperature calcined in addition to elemental carbon, includes commercial macarbon beneath the composite cathode material results in terials that are normally employed as organic sources amore stable total cell bottom structure. of carbon or derived from organic sources, for example: It is apparent from the above that various changes calcined coke, tars, pitches, coal; graphite; etc.

TABLE I Electrical resistivity (Si-cm.) at room temperature ofsamplcsbaked at Cathodic Wet by Sample Composition instability molten No. 900 C. 1,125 O. 1,270 C. 1,350" 0. 1,575" O. l,800 C. percent aluminum expansion 1 Avg. carbon cell lining 1 6. 5 o. 2.. 90% anthracite, RI-IS 2 2.9 Partially 3. 80% anthracite, RHS- 4. 5 Do. 4.- 90% anthracite, 10% RI-IS 1.1 Do. 5.. M. 50% anthracite, 50% RHS 1. 5 Yes. 6..- 20% anthracite, 80% RHS 2. 6 Yes 7 80% pet. coke, 20% RHS. 1.0 Yes 8.. H. 50% pet. coke, 50% EH8"- 1.0 Yes 9 20% pet. coke, 80% R118 1.3 Yes 1 Formed of carbon which has been calcined at about 1,200 C. 2 RHSzRefrnctory hard substance. 3 Cathodic expansion as determined by the procedure described herein. 4 Anthracite calcined at 1,200 C. An additional advantage of the invention is that the and modifications may be made without departing from cell design can 'be readily adaptable into conventional the invention. cells in existing plants. Although the design is particu- What is claimed is: larly suited to conventional cells using pre'baked anodes 1. An electrolytic cell having a shell defining a recepit may be also employed in cells using self baking or tacle, an anodic system including at least one anode sus- Soderberg anodes. In addition, although the drained cathpended within the shell and a cathodic system comprisodes are shown in FIGURE 1 as sloping toward the ing a cathodic structure disposed within said receptacle center of the cells it is possible that cathodes may be and adapted to be exposed to molten constituents during sloped in ditferent directions if drains or suitable pass'agecell operation, said cathodic structure comprising at least ways for the flow of molten aluminum leading to a one drained, molten-constituent contacting surface, said molten metal collection zone are provided. Similarly molten-constituent contacting surface comprising coma single sloping surface or a plurality of sloping surfaces posite cathode material which has a cathodic expansion may be used. Furthermore, as discussed above other con- 40 measured by the procedure set forth in the specification of figurations may be employed which provide a dra d less than about 3% and is wettable by molten aluminum, and wetted cathode surface and in which current is passed aid o o it thod t rial comprising a i t of from the anode through the electrolyte, through a subrefractory h d metal and at least about 5% carbon, stantially thin film 0f lTlOitCIl aluminum and llltO the 2 An glectrolytic cell according to claim 1 wherein composite cathode material from which the current flows id composite h d i l h a h di expaninto the cathodic system either directly or through interign f l th bout 15%, mediate Current a 3. An electrolytic cell according to claim 1 wherein In the g, Composite Cathode material 30 is Shown the distance between said anode and said molten constituas extending along the Sides of the Current Collecting \Vellent contacting surface of said cathodic structure is less However, the composite cathode mixture may be emth b t one i h ployed only at the molten constituent contacting surface 4, I an 1 1 n 11 h i a h ll d fi i a if desired. Alternatively, the entire bottom or merely the ceptacle, an anodic system including at least one anode sloping Surface of the cathOde Structure y be formed suspended within the shell and a cathodic system includf h composite cathode material. The composite ing a cathodic structure disposed within said receptacle tum itself y rammed 0r Otherwisfi Packed Over a and which is adapted to be exposed to molten constitucarbon body as shown. M v r, as discussed v pr ents during cell operation, the improvement comprising r d /0r ak ti ns of Composite Ca d a cathodic structure having at least one drained, moltenwrc y be used to form a cathodic structure r l k constituent contacting surface which is comprised of comof composi e ca hode mix re and c rb n may be a posite cathode material, said composite cathode material sembled within the cell. It may also be desirable for comprising a i t f refractory h d t l d t certain arrangements to apply the composite cathode maleast about 5% carbon which is wettable by molten alumiterial in decreasing concentration away from the molten u nd h a ath di expansion measured b h constituent contacting surface. Thus, for example, a cacedure set forth in the specification of less than about 3%, th ru tu m y e H d n w i h t lt n c nand a molten aluminum collection zone positioned within stituent contacting surface consists essentially of the com- 5 aid receptacle so th t id 11 t d lt l i posite cathode material Whilfi the interior f CathOdiC does not form an essential part of the electric circuit. Structure y he Composed of a mixture of a gradually 5. In an electrolytic cell having a shell defining a reor incrementally diminishing amounts of composite cathceptacle, an anodic system including at least one anode ode material. The proportion of composite cathode matesuspended withi th h ll, a th di System comprising rial in the cathodic structure may decrease from suba cathodic structure disposed within said receptacle and stantially 100% at the molten constituent contacting surhi h i adapted t b exposed t molten Constituents face to as low as 0% at the bottom of the cathodic strucduring cell operation, the improvement comprising a cature. thodic structure having at least one sloping molten con- As indicated above the presently preferred technique stituent contacting surface comprising a mixture of at for producing a cathodic structure having a drained and least one refractory hard metal and at least about 5% carbon, said mixture having a cathodic expansion measured by the procedure set forth in the specification of less than about 3% and being wettable by molten aluminum.

6. In an electrolytic cell having a shell defining a receptacle, an anodic system including at least one anode suspended within the shell, a cathodic system comprising a cathodic structure disposed within said receptacle and which is adapted to be exposed to molten constituents during cell operation, the improvement comprising a cathodic structure having at least one sloping, molten-com stituent contacting surface comprising composite cathode material which comprises a mixture baked at at least 900 C. of refractory hard metal and at least about 5% carbon which has been calcined at a temperature of at least 1500 C., said composite cathode material being wettable by molten aluminum and having a cathodic expansion measured by the procedure set forth in the specification of less than about 3%.

7. An improvement according to claim 6 wherein said composite cathode material has a cathodic expansion of less than about 1.5%.

8. A cathodic structure for use in an electrolytic cell having a molten-constituent contacting surface, said molten-constituent contacting surface comprising a mixture of refractory hard metal and at least about 5% carbon, said mixture being wettable by molten aluminum and having a cathodic expansion measured by the procedure set forth in the specification of less than about 3%.

9. A cathodic structure according to claim 8 wherein said carbon component of said mixture comprises carbonaceous material which has been heat treated above about 1500 C. to improved its stability under cathodic conditions in an electrolytic cell.

10. A method of operating an electrolytic cell for the production of aluminum which comprises a shell defining a receptacle, electrolyte containing dissolved aluminum compound within said receptacle, an an-odic system including at least one anode suspended within the shell, and a cathodic system including a drained cathodic structure comprising a mixture of refractory hard metal and at least about 5% carbon, wettable by molten aluminum and having a cathodic expansion measured by the procedure set forth in the specification of less than about 3% disposed within said receptacle and which is adapted to be exposed to molten constituents during cell operation; comprising connecting said anode and cathodic structure to a source of electric power such that current passes through the anode and electrolyte causing a substantially thin film of molten aluminum to form on said cathodic structure and then passes through said thin film of molten aluminum into said cathodic structure.

11. A method according to claim 10 wherein said electrolytic cell is operated with an interpolar spacing of less than about one inch.

12. A method according to claim 11, wherein said cell is operated with a current efficiency of at least about 80%.

13. A method of making a composite cathode surface material for a cathodic structure of an electrolytic cell comprising forming a mixture of refractory hard metal and at least about 5% carbon, and baking the mixture at a temperature of at least about 900 C. to produce a rigid mass wettable by molten aluminum and having a cathodic expansion measured by the procedure set forth in the specification of less than about 3%.

14. A method according to claim 9 wherein said carbon component comprises carbonaceous material which has been heat treated at a temperature of 1600-2000 C.

15. A method according to claim 9 wherein up to 20% binder is used.

16. A method according to claim 9 wherein said mixture is baked at a temperature of 900-1800 C.

17. A method of making a cathodic structure for an electrolytic cell comprising preparing a green mixture of at least about 5% carbon which has been calcined at a temperature of at least about 1500 C., refractory hard metal and a binder, disposing said mixture Within an electrolytic cell, forming at least one sloping molten-constituent contacting surface from said mixture and baking said mixture at a temperature of at least about 900 C. to produce a rigid mass wettable by molten aluminum and having a cathodic expansion of less than about 3%.

18. A method according to claim 17 wherein 10-20% carbonaceous binder is mixed with said green mixture.

19. A method of making a cathodic structure wettable by molten aluminum and having a cathodic expansion measured by the procedure set forth in the specification of less than about 3% for an electrolytic cell for the production of aluminum comprising preparing a mixture of at least about 5% carbon which has been calcined at a temperature of at least about 1500 C., a refractory hard metal and a binder, forming said mixture into bodies, baking said bodies at a temperature of at least about 900 C., and assemblying said bodies to form a sloping structure at the bottom of said cell.

20. A method of making a cathodic structure wettable by molten aluminum and having a cathodic expansion measured by the procedure set forth in the specification of less than about 3% for an electrolytic cell for the production of aluminum comprising preparing a mixture of at least about 5% carbon which has been calcined at a temperature of at least about 1500 C., refractory hard metal and a binder, forming said mixture into a body with a sloping surface, baking said body at a temperature of at least about 900 C. and disposing said body within said cell.

References Cited UNITED STATES PATENTS 2,480,474 8/ 1949' Johnson 204-67 2,636,856 4/1953 Suggs et al. 204291 3,011,982 12/1961 Maduk et al 204291 3,202,600 8/1965 Ransley 204-243 FOREIGN PATENTS 150,293 2/ 1953 Australia.

JOHN H. MACK, Primary Examiner.

D. R. VALENTINE, Assistant Examiner. 

